Photoconductive substances widely known as materials for electrophotographic photoreceptors include inorganic substances, such as amorphous selenium, selenium alloys, cadmium sulfide, zinc oxide, etc., and organic substances exemplified by polyvinylcarbazole and its derivatives. The organic photoconductive substances are advantageous over the inorganic ones because of superiority in transparency, film-forming properties, flexibility, productivity, and the like.
In spite of the above-mentioned advantage, the organic photoconductive substances have not been used in an electrophotographic photoreceptor since they are inferior to the inorganic photoconductive substances in sensitivity and durability.
In an attempt of overcoming these problems, electrophotographic photoreceptors having a photosensitive layer containing an azo pigment have been proposed, such as those disclosed in Japanese patent application (OPI) Nos. 144358/83, 194035/83, 36253/84, 146062/84, 196770/85, and 189759/85 (the term "OPI" as used herein means an "unexamined published Japanese patent application"). However, none of those azo pigments used therein is satisfactory.